U.S. patent application number 14/111984 was filed with the patent office on 2014-05-29 for multilevel converter and method of starting up a multilevel converter.
The applicant listed for this patent is SIEMENS AKTIENGESELLSCHAFT. Invention is credited to Anandarup Das, Hamed Nademi, Lars Norum.
Application Number | 20140146586 14/111984 |
Document ID | / |
Family ID | 45952546 |
Filed Date | 2014-05-29 |
United States Patent
Application |
20140146586 |
Kind Code |
A1 |
Das; Anandarup ; et
al. |
May 29, 2014 |
MULTILEVEL CONVERTER AND METHOD OF STARTING UP A MULTILEVEL
CONVERTER
Abstract
A multilevel converter for converting between an AC voltage and
a DC voltage and a method of starting up such multilevel converter
are provided. The multilevel converter has an AC terminal and a DC
terminal for connecting the multilevel converter to either an AC
power source or a DC power source, respectively, which supplies the
voltage to be converted. The multilevel converter further comprises
at least one converter leg, the DC terminal comprising a first and
a second DC terminal, the converter leg comprising plural converter
cells connected in series between the first and second DC
terminals. The AC terminal of the multilevel converter is
electrically coupled to an electrical link between two of said
converter cells of said converter leg.
Inventors: |
Das; Anandarup; (Trondheim,
NO) ; Nademi; Hamed; (Trondheim, NO) ; Norum;
Lars; (Trondheim, NO) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SIEMENS AKTIENGESELLSCHAFT |
Munchen |
|
DE |
|
|
Family ID: |
45952546 |
Appl. No.: |
14/111984 |
Filed: |
April 10, 2012 |
PCT Filed: |
April 10, 2012 |
PCT NO: |
PCT/EP2012/056440 |
371 Date: |
February 10, 2014 |
Current U.S.
Class: |
363/49 ; 320/166;
363/13 |
Current CPC
Class: |
H02M 2007/4835 20130101;
H02M 1/36 20130101; H02M 7/68 20130101; H02M 7/49 20130101 |
Class at
Publication: |
363/49 ; 363/13;
320/166 |
International
Class: |
H02M 7/68 20060101
H02M007/68; H02M 1/36 20060101 H02M001/36 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 15, 2011 |
EP |
11162659.4 |
Claims
1. A multilevel converter for converting between an AC voltage and
a DC voltage, the multilevel converter (10) having an AC terminal
(15) and a DC terminal (11, 12) for connecting the multilevel
converter to either an AC power source (14) or a DC power source
(13), respectively, which supplies the voltage to be converted,
wherein the multilevel converter (10) comprises: at least one
converter leg (21), the DC terminal comprising a first DC terminal
(11) and a second DC terminal (12), the converter leg (21)
comprising plural converter cells (30, 31, 32) connected in series
between the first and second DC terminals (11, 12), wherein the AC
terminal (15) of the multilevel converter is electrically coupled
to an electrical link (41) between two of said converter cells (30,
31, 32) of said converter leg (21), and a resistor circuit (71, 72)
comprised in said converter leg (21) and connected in series with
said converter cells (30, 31, 32), the resistor circuit (71, 72)
being configured so as to be capable of connecting a resistance
(R.sub.C1, R.sub.C2) in series with the converter cells (30, 31,
32) of the converter leg (21), wherein each converter cell (30, 31,
32) comprises a capacitor (55), the multilevel converter (10) being
configured so as to enable a charging of the capacitor (55) of a
converter cell (30, 31, 32) from the power source connected to the
AC terminal (15) or the DC terminal (11, 12) of the multilevel
converter through the resistance of the resistor circuit (71,
72).
2. The multilevel converter according to claim 1, wherein the
resistor circuit (71, 72) comprises said resistance (R.sub.C1,
R.sub.C2) connected in series with the converter cells, the
resistor circuit further comprising a switch (S.sub.C1, S.sub.C2)
connected in parallel with said resistance (R.sub.C1, R.sub.C2) so
as to enable a bypassing of the resistance (R.sub.C1, R.sub.C2) by
closing the switch (S.sub.C1, S.sub.C2).
3. The multilevel converter according to claim 2, wherein the
switch (S.sub.C1, S.sub.C2) is a mechanical switch, an electronic
switch or a semiconductor switch.
4. The multilevel converter according to claim 1, wherein the
converter leg (21) comprises a first converter arm (61) comprising
the converter cells (30, 31) coupled between the first DC terminal
(11) and the electrical link (41) and a second converter arm (62)
comprising the converter cells (32, 33) coupled between the second
DC terminal (12) and the electrical link (41), wherein the resistor
circuit (71) is comprised in the first converter arm (61), the
second converter arm (62) further comprising a second resistor
circuit (72) connected in series with the converter cells (32, 33)
of the second converter arm (62).
5. The multilevel converter according to claim 1, wherein the
multilevel converter (10) is adapted so as to be capable of
discharging the capacitor (55) of a converter cell (30, 31, 32)
through the resistance (R.sub.C1, R.sub.C2) of the resistor circuit
(71, 72).
6. The multilevel converter according to claim 1, further
comprising a switch (80) to connect the AC terminal (15) of a first
of the converter legs (21) to a second AC terminal (16) of a second
of the converter legs (22) so as to enable the discharging of the
capacitor (55) of a converter cell through at least part of the
first and the second converter legs (21, 22).
7. The multilevel converter according to claim 1, wherein the
converter leg (21) further comprises an inductance (18) connected
in series with the converter cells (30, 31, 32), the resistance of
the resistor circuit (71, 72) being configured such that the series
connection of the inductance (18), the resistance (R.sub.C1,
R.sub.C2) and the capacitor (55) of one converter cell (30) or of
the converter cells (30, 31) connected between the first or second
DC terminal (11, 12) and the electrical link (41) provide an
overdamped system.
8. The multilevel converter according to claim 1, wherein each
converter cell comprises two terminals (56, 57) by which the
converter cell (30) is connected in series with the other converter
cells (31, 32) in the converter leg (21), the converter cell
comprising a first switch (S.sub.1) and a second switch (S.sub.2),
the second switch (S.sub.2) being connected in series with the
capacitor (55) of the converter cell, the first switch (S.sub.1)
being connected in parallel with the capacitor (55) and the second
switch (S.sub.2) of the converter cell (30).
9. The multilevel converter according to claim 1, wherein the
converter leg (21) comprises a first converter arm (61) comprising
the converter cells (30, 31) coupled between the first DC terminal
(11) and the electrical link (41) and a second converter arm (62)
comprising the converter cells (32, 33) coupled between the second
DC terminal (12) and the electrical link (41), and wherein the
multilevel converter is configured so that by closing all first
switches of the converter cells in the second converter arm and by
opening all first switches (S.sub.i1) and closing all second
switches (S.sub.i2) of the converter cells in the first converter
arm (61), the capacitor (55) of each converter cell in the first
converter arm (61) can be charged through the resistance.
10. A method of starting up a multilevel converter, the multilevel
converter (10) being adapted to convert between an AC voltage and a
DC voltage, the multilevel converter having an AC terminal (15) and
a DC terminal (11, 12) for connecting the multilevel converter to
either an AC power source (14) or a DC power source (13),
respectively, which supplies the voltage to be converted, the
multilevel converter comprising a converter leg (21) with plural
converter cells (30, 31, 32) connected in series, each converter
cell comprising a capacitor (55), the method comprising the steps
of supplying electric power of the AC power source or the DC power
source to the converter leg (21), connecting a resistance
(R.sub.C1, R.sub.C2) in series with the converter cells (30, 31,
32), and connecting the capacitor (55) of one or more converter
cells (30, 31) in series with the resistance (R.sub.C1, R.sub.C2),
wherein the one or more capacitors (55) connected in series with
the resistance are charged from the connected power source (13, 14)
through the resistance (R.sub.C1, R.sub.C2).
11. The method according to claim 10, wherein the step of
connecting the capacitor (55) of one or more converter cells (30,
31, 32) in series with the resistance (R.sub.C1, R.sub.C2) so as to
charge the one or more capacitors through the resistance is
repeated until the capacitor of each converter cell in the
converter leg (21) is charged.
12. The method according to claim 10, wherein the converter leg
(21) comprises a first converter arm (61) comprising the converter
cells (30, 31) coupled between the first DC terminal (11) and the
electrical link (41) and a second converter arm (62) comprising the
converter cells (32, 33) coupled between the second DC terminal
(12) and the electrical link (41), wherein the step of connecting
the capacitor (55) of one or more converter cells in series with
the resistance (R.sub.C1, R.sub.C2) comprises connecting the
capacitor (55) of each converter cell in the first or second
converter arm (61, 62) in series with the resistance (R.sub.C1,
R.sub.C2), so that each of the capacitors (55) of the respective
converter arm (61, 62) is charged through the resistance (R.sub.C1,
R.sub.C2) from the connected power source.
13. The method according to claim 10, wherein at the DC terminal
(11, 12) of the multilevel converter, a predetermined DC voltage is
supplied by a DC power source or is to be supplied to a DC load,
wherein the converter leg (21) comprises a first converter arm (61)
comprising the converter cells (30, 31) coupled between the first
DC terminal (11) and the electrical link (41) and a second
converter arm (62) comprising the converter cells (32, 33) coupled
between the second DC terminal (12) and the electrical link (41),
wherein for each converter arm (61, 62), the capacitor of each
converter cell of the converter arm is charged to a voltage that is
about equal to the predetermined DC voltage (V.sub.dc) divided by
the number of converter cells (n) comprised in the respective
converter arm (61, 62).
14. The method according to claim 10, wherein the at least one
converter leg comprises a first converter leg (21) for a first
phase of the AC voltage and a second converter leg (22) for a
second phase of the AC voltage, the first and second converter legs
being connected in parallel between the first and second DC
terminals (11, 12) of the multilevel converter, the method further
comprising connecting a first phase of the AC power supply to the
AC terminal (15) of the first converter leg (21) and connecting a
second phase of the AC power supply to an AC terminal (16) of the
second converter leg (22), connecting the capacitor (55) of at
least one converter cell (30) of the first converter leg (21)
between one DC terminal (11, 12) and an AC terminal (15) of the
first converter leg (21), connecting the resistance (R.sub.C1) of
the first converter leg (21) in series with the capacitor (55) of
the at least one converter cell, and providing an electrical
connection (80) between the AC terminal (16) of the second
converter leg and said DC terminal (11, 12) by means of the second
converter leg (22), wherein the at least one capacitor (55) is
charged from the AC power source (14) via at least part of the
first and second converter legs (21, 22).
15. A method of discharging a capacitor (55) of a multilevel
converter (10), the multilevel converter being adapted to convert
between an AC voltage and a DC voltage, the multilevel converter
comprising at least a first and a second converter leg (21, 22)
each being connected between a first DC terminal (11) and a second
DC terminal (12) of the multilevel converter (10), wherein each the
converter legs (21, 22) comprises plural converter cells (30, 31,
32) connected in series between the first and second DC terminals
(11, 12), each converter cell comprising a capacitor (55), the
method comprising the steps of disconnecting the power source and a
load from the multilevel converter (10), connecting the capacitor
(55) of a converter cell (30, 31, 32) comprised in one of the
converter legs (21, 22) in series with the converter cells of the
converter leg, connecting a resistance (R.sub.C1, R.sub.C2) in
series with the capacitor (55) of the converter cell (30, 31, 32),
and providing an electrical connection (80) between the first and
second converter legs (21, 22) such that the capacitor (55) is
discharged through the resistance (R.sub.C1, R.sub.C2), at least a
part of the first and second converter legs (21, 22), and said
electrical connection (80).
Description
FIELD OF THE INVENTION
[0001] The invention relates to a multilevel converter and to a
method of starting up a multilevel converter. In particular, it
relates to the charging of a capacitor of a converter cell of a
multilevel converter.
BACKGROUND OF THE INVENTION
[0002] Multilevel converters are now frequently being employed for
converting between an AC (alternating current) voltage and a DC
(direct current) voltage. Such converters provide different voltage
levels by which an AC voltage can for example be synthesized. The
converter may further use a pulse width modulation (PWM) technique
in the generation of the AC voltage. The use of different voltage
levels at the AC output further reduces the required switching
frequency for PWM.
[0003] The Modular Multilevel Converter (MMC) is a promising
multilevel converter topology proposed in recent times. Such
converter has a modular structure, which may provide redundant
cells for fault tolerant applications and an easy scalability. A
MMC can comprise a number of converter cells in series. Each cell
can have of two switches and a capacitor. When one of the switches
is turned on, the capacitor is bypassed and the output voltage of
the converter cell is zero. When the other switch is turned on, the
capacitor voltage is obtained at the output. With many cells
connected in series, the output voltage of the converter can be
made smooth and no or very minimal filtering is required to improve
the output voltage quality.
[0004] The starting up such converter, e.g. from a de-energized
state in which the capacitors of the converter cells are
substantially discharged, is generally difficult. A `black start`
of the converter for reaching the operation conditions may be
performed by using an auxiliary voltage source having an output
voltage similar to the nominal voltage of the capacitor of a
converter cell in order to individually charge each of the
capacitors of the plurality of converter cells. Such procedure may
be time consuming and requires an additional auxiliary voltage
source.
SUMMARY
[0005] Accordingly, there is a need to improve the start-up of a
multilevel converter, in particular to improve the charging of a
capacitor of a converter cell of the multilevel converter.
[0006] This need is met by the features of the independent claims.
The dependent claims describe embodiments of the invention.
[0007] An aspect of the invention relates to a multilevel converter
for converting between an AC voltage and a DC voltage. The
multilevel converter has an AC terminal and a DC terminal for
connecting the multilevel converter to either an AC power source or
a DC power source, respectively, which supplies the voltage to be
converted. The multilevel converter comprises at least one
converter leg. The DC terminal comprises a first and a second DC
terminal, the converter leg comprising plural converter cells
connected in series between the first and second DC terminals. The
AC terminal of the multilevel converter is electrically coupled to
an electrical link between two of the converter cells of said
converter leg. The multilevel converter further comprises a
resistor circuit comprised in said converter leg and connected in
series with the converter cells. The resistor circuit is configured
so as to be capable of connecting a resistance in series with the
converter cells of the converter leg. Each converter cell comprises
a capacitor. The multilevel converter is configured so as to enable
a charging of the capacitor of a converter cell from the power
source connected to the AC terminal or the DC terminal of the
multilevel converter through the resistance of the resistor
circuit.
[0008] With such configuration, the capacitors of the converter
cells may be charged at startup directly from the main power source
(or operating power source), i.e. from the AC power source or the
DC power source which supplies the electrical power to be converted
during operation of the multilevel converter. No auxiliary power
source is thus needed for charging the capacitors of the converter
cells at start-up. The start-up of the multilevel converter may
thus be facilitated and accelerated.
[0009] In an embodiment, the resistor circuit comprises the
resistance connected in series with the converter cells. It may
further comprise a switch connected in parallel with the resistance
so as to enable a bypassing of the resistance by closing the
switch. An efficient way of inserting the resistance into the
converter leg circuit and of removing it therefrom is thus
provided.
[0010] Other configurations, in which the resistor circuit may for
example comprise an adjustable resistance that can be adjusted to a
value of about zero, are also conceivable.
[0011] The switch may be a mechanical switch, an electronic switch
or a semiconductor switch. The switch may be operated by a
controller.
[0012] The resistor circuit may be connected between a converter
cell and the electrical link towards which the AC terminal is
coupled.
[0013] In an embodiment, the converter leg may comprise a first
converter arm comprising the converter cells coupled between the
first DC terminal and the electrical link, and a second converter
arm comprising the converter cells coupled between the second DC
terminal and the electrical link. The resistor circuit may be
comprised in the first converter arm. The second converter arm may
further comprise a second resistor circuit connected in series with
the converter cells of the second converter arm. The resistance
provided by the resistor circuit may thus be controlled
individually for each converter arm.
[0014] The second resistor circuit may be substantially similar to
the first resistor circuit.
[0015] The multilevel converter may operate as a rectifier that is
supplied with a one phase or three phase AC voltage via the AC
terminal and generates a DC voltage on the DC terminals. It may
also operate as an inverter that generates from a DC voltage
supplied to the DC terminals a one phase or three phase AC
voltage.
[0016] In an embodiment, the at least one converter leg may
comprise three converter legs, each converter leg being coupled to
an AC terminal for supplying or receiving an AC voltage of a
different phase. The multilevel converter can be configured to
convert to or from a three phase AC voltage.
[0017] The multilevel converter may be adapted so as to be capable
of discharging the capacitor of a converter cell through the
resistance of the resistor circuit. The multilevel converter may
thus be brought safely into a de-energized state.
[0018] The multilevel converter may further comprise a switch to
connect the AC terminal of a first of the converter legs to a
second AC terminal of a second of the converter legs so as to
enable the discharging of the capacitor through at least part of
the first and the second converter legs. When power source and load
are disconnected from the multilevel converter, it may thus safely
be brought into the de-energized state without requiring auxiliary
equipment.
[0019] The converter leg may further comprise an inductance
connected in series with the converter cells. A smooth output
voltage may thus be obtained.
[0020] In an embodiment, the converter leg further comprises an
inductance connected in series with the converter cells, the
resistance of the resistor circuit being configured such that the
series connection of the inductance, the resistance and the
capacitor of one converter cell or of the converter cells connected
between the first or second DC terminal and the electrical link
provide an overdamped system. Overshoot, in particular overcurrents
may thus be avoided when the capacitor of the converter cell is
charged from the main AC or DC power supply.
[0021] Each converter cell may comprise two terminals by which the
converter cell is connected in series with the other converter
cells in the converter leg. The converter cell may comprise a first
switch and a second switch. The second switch can be connected in
series with the capacitor of the converter cell. The first switch
can be connected in parallel with the capacitor and the second
switch of the converter cell. Thus, by closing the first switch,
the second switch and the capacitor may be bypassed. The capacitor
of the converter cell may thus efficiently be inserted or taken out
of the circuitry of the converter leg.
[0022] The switches may be semiconductor switches. The switches may
be selected from a group comprising an insulated gate bipolar
transistor (IGBT), a power MOSFET, a power thyristor (like a SCR, a
GTO, a MCTs or the like), and a bipolar junction transistor
(BJT).
[0023] In an embodiment, the converter leg comprises a first
converter arm comprising the converter cells coupled between the
first DC terminal and the electrical link and a second converter
arm comprising the converter cells coupled between the second DC
terminal and the electrical link. The multilevel converter may be
configured so that by closing all first switches of the converter
cells in the second converter arm and by opening all first switches
and closing all second switches of the converter cells in the first
converter arm, the capacitor of each converter cell in the first
converter arm is charged. As all capacitors of the converter arm
can be charged simultaneously, the charging process can be further
accelerated and simplified. Note that the configuration may be such
that a similar charging of the capacitors in the second converter
arm is enabled, e.g. by exchanging the switching state of the
switches of the first and second converter arms.
[0024] As the cells are connected in series, the capacitors of the
cells of a converter arm can be connected in series as well. The
multilevel converter can thus be configured so that by applying a
voltage to a converter arm, each capacitor in the converter arm is
charged by said voltage divided by the number of capacitors
connected in series in the converter arm. The applied voltage is
for example the DC voltage of the DC power source provided at the
DC terminal.
[0025] Charging may also occur by the AC voltage of an AC power
source provided at the AC terminal. The multilevel converter may
for example be configured such that the one or more capacitors are
connected into the converter leg for charging during a particular
phase of the waveform of the AC voltage, e.g. during the positive
leading edge of the AC voltage waveform.
[0026] In other embodiments, the multilevel converter may comprise
sensors for determining the charging state of the capacitors of the
converter cells. The multilevel converter may then be configured so
as to control the charging of each capacitor to a predetermined
voltage level. For example when charging from an AC power source,
the AC voltage of which has due to the AC waveform varying voltage
levels, it becomes possible to still charge each capacitor to the
desired voltage level. The capacitor may for example be taken out
of the converter arm/leg by means of the above mentioned switches
when the desired charging state is reached.
[0027] The first and second converter arms may comprise the same
number of converter cells. Each converter arm may comprise one of
the above mentioned resistor circuits. The converter arms may be
symmetric with respect to the resistor circuit and an inductance
which can be provided in series connection in the converter
arm.
[0028] In an embodiment, the multilevel converter may further
comprise for each DC terminal a switch (or breaker) for
disconnecting the respective DC terminal from a DC power source or
a load. It may further comprise for each AC terminal a switch (or
breaker) for disconnecting the respective AC terminal from an AC
power source or a load. The power source and the load may thus be
disconnected from the converter, so as to enable the de-energizing
of the converter, e.g. by discharging the capacitors of the
converter cells. For starting up the converter, the power source
may first be connected, the switches of the converter cells may be
set in the above described manner for charging the capacitors,
thereby energizing the converter. The load may after charging be
connected to the converter, and the operation of the converter can
be started.
[0029] The load may for example be a drive, and the multilevel
converter may convert the electrical power provided by the power
source to the electrical power required for operating the drive.
The power source may for example be a DC bus, the converter
converting the DC voltage into an AC voltage for operating an AC
electrical motor of a drive. In other applications, the multilevel
converter may be connected between different types of power grids.
It may convert an AC voltage provided by an AC power grid into a DC
voltage for DC electric power transmission, such as HVDC (high
voltage DC).
[0030] In an embodiment, the multilevel converter may be a modular
multilevel converter (MMC), and the converter cells may be
converter modules of the modular multilevel converter.
[0031] Another aspect of the invention relates to a method of
starting up a multilevel converter. The multilevel converter is
adapted to convert between an AC voltage and a DC voltage, the
multilevel converter having an AC terminal and a DC terminal for
connecting the multilevel converter to either an AC power source or
a DC power source, respectively, which supplies the voltage to be
converted. The multilevel converter comprises a converter leg with
plural converter cells connected in series. Each converter cell
comprises a capacitor. The method comprises the steps of supplying
electric power of the AC power source or the DC power source to the
converter leg, connecting a resistance in series with the converter
cells, and connecting the capacitor of one or more converter cells
in series with the resistance. This is performed such that the one
or more capacitors connected in series with the resistance are
charged from the connected power source through the resistance.
[0032] With the method, similar advantages as the ones outlined
above with respect to the multilevel converter may be achieved. In
particular, the one or more capacitors can be charged directly from
the main power source, so that no auxiliary power source is
required. Note that the order of the method steps can be changed,
i.e. the resistor and capacitor may first be connected in series
whereafter the electric power is supplied to the converter leg.
[0033] In an embodiment, the step of connecting the capacitor of
one or more converter cells in series with the resistance so as to
charge the one or more capacitors through the resistance is
repeated until the capacitor of each converter cell in the
converter leg is charged.
[0034] In some embodiments, each capacitor may be individually
connected in series with the resistance and charged therethrough,
i.e. one after the other. In other embodiments, plural capacitors
may be connected in series with the resistance and charged
simultaneously. As outlined above, a feedback system determining
the charging state of each capacitor may be provided so that each
capacitor can be charged to a predetermined voltage level. This is
particularly advantageous when charging from an AC power
source.
[0035] In an embodiment, the converter leg may comprise a first
converter arm comprising the converter cells coupled between the
first DC terminal and the electrical link and a second converter
arm comprising the converter cells coupled between the second DC
terminal and the electrical link. The step of connecting the
capacitor of one or more converter cells in series with the
resistance may comprise connecting the capacitor of each converter
cell in the first or the second converter arm in series with the
resistance, so that each of the capacitors of the respective
converter arm is charged through the resistance from the AC power
source or the DC power source. This can for example be performed
for the first converter arm and subsequently for the second
converter arm, so that with two charging operations, each of said
capacitors in the converter leg can be charged.
[0036] The electrical link at which the AC terminal is connected
can be the electrical link between the first and the second
converter arms. Each converter arm can comprise one of the above
mentioned resistor circuits for connecting the resistance in series
with the converter cells of each arm. The capacitors may be charged
through the resistances of both resistor circuits in the converter
leg.
[0037] The multilevel converter may comprise further converter
legs, e.g. three converter legs for providing/receiving three phase
AC electrical power. Each converter leg can be configured as
outlined above, with two converter arms and one or two resistor
circuits.
[0038] For discharging a capacitor, the capacitor can be connected
in series with the converter cells and the electrical links of two
converter legs can be connected to each other, so that the
capacitor is discharged through the two first converter arms or the
two second converter arms of the two converter legs (in dependence
on in which arm the capacitor is located).
[0039] In an embodiment, a predetermined DC voltage is supplied by
a DC power source or is to be supplied to a DC load at the DC
terminal of the multilevel converter. The converter leg may
comprise a first converter arm comprising the converter cells
coupled between the first DC terminal and the electrical link and a
second converter arm comprising the converter cells coupled between
the second DC terminal and the electrical link. For each converter
arm, the capacitor of each converter cell of the converter arm can
be charged to a voltage that is about equal to the predetermined DC
voltage divided by the number of converter cells comprised in the
respective converter arm. With n converter cells in each converter
arm, each capacitor may for example be charged up to a voltage of
V.sub.dc/n, wherein V.sub.dc is the predetermined voltage between
the first and second DC terminals. This charging level may be
suitable for starting the operation of the multilevel
converter.
[0040] In a further embodiment, the at least one converter leg
comprises a first converter leg for a first phase of the AC voltage
and a second converter leg for a second phase of the AC voltage,
the first and second converter legs being connected in parallel
between the first and second DC terminals of the multilevel
converter. The method may further comprise connecting a first phase
of the AC power supply to the AC terminal of the first converter
leg and connecting a second phase of the AC power supply to an AC
terminal of the second converter leg, connecting the capacitor of
at least one converter cell of the first converter leg between one
DC terminal and an AC terminal of the first converter leg,
connecting the resistance of the first converter leg in series with
the capacitor of the at least one converter cell, and providing an
electrical connection between the AC terminal of the second
converter leg and said DC terminal by means of the second converter
leg. This is performed such that the at least one capacitor is
charged from the AC power source via at least part of the first and
second converter legs. At least part of the second converter leg
may thus function as a `return path` for the AC voltage supplied to
the first converter leg.
[0041] In other embodiments, the method may comprise further steps,
such as disconnecting the load from the multilevel converter before
connecting power supply thereto, i.e. before charging, or removing
the resistance from the series connection with the converter cells
and connecting a load to the multilevel converter after the one or
more capacitors were charged.
[0042] Another aspect of the invention provides a method of
discharging the capacitors of a multilevel converter, the
multilevel converter being adapted to convert between an AC voltage
and a DC voltage, the multilevel converter comprising at a first
and a second converter leg each being connected between a first DC
terminal and a second DC terminal of the multilevel converter,
wherein each the converter legs comprises plural converter cells
connected in series between the first and second DC terminals, and
wherein each converter cell comprising a capacitor. The method
comprises the steps of disconnecting the power source and a load
from the multilevel converter, connecting the capacitor of a
converter cell comprised in one of the converter legs in series
with the converter cells of the converter leg, connecting a
resistance in series with the capacitor of the converter cell, and
providing an electrical connection between the first and second
converter legs such that the capacitor is discharged through the
resistance, at least a part of the first and second converter legs,
one of said (disconnected) DC terminals and said electrical
connection.
[0043] By connecting the resistance in series with the capacitor,
discharging of the capacitor is facilitated and can be performed by
simple switching operations. No auxiliary equipment is necessary to
discharge the capacitor. The multilevel converter can thus safely
be brought into a de-energized state.
[0044] The method may be carried out by the above described
multilevel converter or embodiments thereof. In particular,
connecting the resistance in series with the capacitor of a
converter cell may be performed by the above mentioned resistor
circuit.
[0045] The features of the aspects and embodiments of the invention
mentioned above and those yet to be explained below can be combined
with each other unless noted to the contrary.
[0046] In particular, the above described methods may be performed
by embodiments of the multilevel converter. Also, the multilevel
converter may be adapted so as to perform any of the methods
described above or embodiments of these.
BRIEF DESCRIPTION OF THE DRAWINGS
[0047] The foregoing and other features and advantages of the
invention will become further apparent from the following detailed
description read in conjunction with the accompanying drawings.
[0048] In the drawings, like reference numerals refer to like
elements.
[0049] FIG. 1 is a schematic diagram illustrating a multilevel
converter.
[0050] FIG. 2 is a schematic diagram illustrating the configuration
of a converter cell that may be used in embodiments of the
invention.
[0051] FIGS. 3A and 3B are schematic diagrams of a multilevel
converter according to an embodiment of the invention.
[0052] FIG. 4 is a schematic diagram illustrating the multilevel
converter of FIG. 3 in a state in which the capacitors of a
converter arm are charged simultaneously.
[0053] FIG. 5 is a schematic diagram illustrating a multilevel
converter according to an embodiment of the invention in a state in
which the capacitor of a converter cell is discharged.
[0054] FIG. 6 is a schematic diagram illustrating a multilevel
converter according to an embodiment of the invention in a state in
which the capacitor of a converter cell is charged from an AC power
source.
[0055] FIG. 7 is a diagram illustrating a peak current in a
converter arm during charging in dependence on a damping factor
that is adjustable by the resistance.
[0056] FIGS. 8a and 8b are diagrams illustrating the subsequent
charging of capacitors of converter cells of a multilevel converter
in accordance with an embodiment of the invention.
[0057] FIGS. 9a and 9b are diagrams illustrating the simultaneous
charging of the capacitors of the converter cells of a converter
arm of a multilevel converter in accordance with an embodiment of
the invention.
[0058] FIG. 10 is a flow diagram illustrating a method according to
an embodiment of the invention.
DETAILED DESCRIPTION
[0059] In the following, the embodiments illustrated in the
accompanying drawings are described in more detail. It should be
clear that the following description is only illustrative and non
restrictive. The drawings are only schematic representations, and
elements in the drawings are not necessarily to scale with each
other. Connections between elements illustrated in the drawings may
be direct or indirect couplings, e.g. couplings with one or more
intervening elements.
[0060] FIG. 1 illustrates a multilevel converter 10, which is
adapted to convert between an AC voltage and a DC voltage.
Multilevel converter 10 comprises a first DC terminal 11 and a
second DC terminal 12 for connecting to a DC power source 13 or to
a load requiring a DC electrical power. Via terminals 11 and 12,
the multilevel converter 10 may for example be connected to DC
transmission line for a HVDC (high voltage DC) transmission of
electrical energy.
[0061] On the AC side of the multilevel converter 10, AC terminals
15, 16 and 17 are provided for connecting a three phase AC power
source or for supplying three phase AC power to an AC load, such as
an AC motor or the like. In other configurations, only one AC
terminal for supplying or receiving a single phase AC voltage may
be provided. When a DC power source is connected to the DC
terminals 11 and 12 and an AC load is connected to the terminals
15, 16 and 17, the multilevel converter 10 operates as an inverter.
Vice versa, when an AC power source is connected to AC terminals
15, 16 and 17 and a DC load is connected to the DC terminals 11 and
12, the multilevel converter 10 operates as a rectifier.
[0062] In the multilevel converter illustrated in FIG. 1, three
converter legs 21, 22 and 23 are connected in parallel between the
DC terminals 11 and 12. Each converter leg comprises a number of
converter cells 30, 31, 32, 33 which are connected in series
between the first DC terminal 11 and the second DC terminal 12. As
indicated by the dots, each converter leg may comprise further
converter cells that can be connected in series. The converter
cells can be provided as modules, and the multilevel converter 10
can be a modular multilevel converter (MMC). Such modular structure
enables scalability and provides a certain redundancy of the
converter cells which may be used for fault tolerant applications.
It should be clear that the multilevel converter 10 may comprise
further converter legs, or may comprise only one or two converter
legs.
[0063] The AC terminals 15, 16 and 17 are connected to the
converter legs 21, 22 and 23 at the electrical links 41, 42 and 43
between adjacent converter cells. For example in converter leg 21,
the electrical link 41 provides an electrical connection between
the converter cells 31 and 32. AC terminal 15 is connected to the
electrical link 41. The upper part of the converter leg (above the
electrical link) can be termed upper converter arm and the lower
part of the converter leg can be termed lower converter arm.
[0064] In each converter arm, an inductance 18 is furthermore
provided. In the example of FIG. 1, the inductance is connected
between the electrical link towards which the AC terminal is
coupled and a converter cell. In other configurations, the
inductance may be connected at a difference position in the
respective converter arm.
[0065] The general description of the multilevel converter 10 given
above is applicable to any of the embodiments described further
below, i.e. embodiments of the multilevel converter 10 described
below may be configured similarly.
[0066] FIG. 2 illustrates an embodiment of a converter cell 30
which may be used with any of the embodiments of the multilevel
converter 10 described herein. Each of the converter cells 30, 31,
32, 33 and the other converter cells of the multilevel converter
may be configured as illustrated in FIG. 2. Converter cell 30
comprises a first switch (S1) 51 and a second switch (S2) 52 and a
capacitor 55. Switches S1 and S2 may be semiconductor switches, in
particular IGBTs, power MOSFETs, power thyristors or the like.
Diodes 58 and 59 are furthermore coupled to the switches 51 and
52.
[0067] The converter cell 30 is connected in series with the other
converter cells of the converter leg using the terminals 56 and 57.
By means of the switches S1 and S2, the voltage at the terminals of
the converter cell 30 can be switched to either zero volt or to the
voltage to which the capacitor 55 is charged. When switch S1 is
turned on, the capacitor is bypassed and the output voltage of the
cell is zero. When S2 is closed, the capacitor voltage is obtained
at the terminals. By connecting several cells in series in the
converter arm, the output voltage of the converter arm can be
adjusted to different voltage levels. Thus, at the AC terminal
connected to the converter leg, different output voltages can be
obtained, and can be adjusted so as to obtain a smooth alternating
voltage that requires no or only a small about of filtering to
improve the output voltage quality.
[0068] Each converter cell 30 may furthermore comprise a control
interface for controlling the switches S1 and S2 and for obtaining
information on the status, in particular the charging state of the
capacitor 55. Such information may be obtained by a voltage sensor.
As an example, a bidirectional fiber optic interface may be
provided in addition to the electric terminals 56 and 57. Thus, by
means of corresponding control software controlling the switches S1
and S2, the charging level of capacitor 55 can be controlled during
the operation of the multilevel converter, and accordingly, the
voltage supplied by the converter cell 30 can be controlled for
generating the required AC output voltage.
[0069] Converter cell 30 may be a converter module (or may be
termed submodule), and converter 10 may be a modular multilevel
converter. In particular, the converter 10 may be configured and
operated as described in the publication "An innovative modular
multilevel converter topology suitable for a wide power range" by
A. Lesnicar and R. Marquardt, in Proc. of IEEE Power Tech Conf.
2003, pp. 1-6, which is incorporated herein by reference in its
entirety.
[0070] With respect to FIGS. 1 and 2, the general operation and
configuration of the converter 10 is described. Modifications of
converter 10 in accordance with embodiments of the invention are
illustrated and described with respect to the remaining FIGS. 3 to
10. Thus, the explanations given above with respect to FIGS. 1 and
2 are equally applicable to the multilevel converters and methods
of starting up such multilevel converter described hereinafter.
[0071] FIGS. 3A and 3B illustrate a configuration of the multilevel
converter 10 having a single converter leg 21, although it should
be clear that further converter legs may be provided. In the upper
converter arm (or first converter arm) 61, a resistor circuit 71 is
provided while in the lower converter arm (or second converter arm)
62, a resistor circuit 72 is provided. Each resistor circuit 71, 72
comprises a resistance R.sub.C1/R.sub.C2 and a switch
S.sub.C1/S.sub.C2 respectively. The resistor circuits 71, 72 are
provided for enabling the charging of the capacitors
C.sub.1-C.sub.8 of the converter cells at start up of the
multilevel converter 10 from the main power source, i.e. either the
DC power source or the AC power source connected to the
converter.
[0072] The switch S.sub.C1/S.sub.C2 of the resistor circuit can be
a mechanical switch or an electronic switch, e.g. a semiconductor
switch. The switching frequency of the switch of the resistor
circuit can be relatively low as switching generally occurs only
during the charging or discharging of the capacitors of the
converter cells. A mechanical contactor may thus be used for this
purpose. By appropriately inserting or bypassing the resistance
R.sub.C1/R.sub.C2 by means of switch SC.sub.1/SC.sub.2,
respectively, the capacitors in the converter cells can be charged
to the desired voltage level. The resistance of the resistor
circuits is also used if the capacitors need to be discharged, e.g.
for the removal of a converter cell from the circuit. This may for
example be necessary if a converter cell needs to be exchanged.
During general operation, i.e. steady state operation of the multi
level converter 10, the resistance R.sub.C1/R.sub.C2 of the
respective resistor circuits can be bypassed by closing the switch
S.sub.C1 and S.sub.C2, respectively.
[0073] Note that FIGS. 3A and 3B illustrate the same multilevel
converter 10, wherein FIG. 3A illustrates a switching state in
which capacitor C.sub.1 is charged, whereas FIG. 3B illustrates a
switching state in which the capacitor C.sub.2 is charged.
[0074] In the present embodiment, the capacitors of the converter
cells 30, 31, . . . are charged from the main voltage source. For
applications in which the converter is used for supplying power to
a drive, the main voltage source may be provided by a DC Bus of a
rectifier system. DC terminals 11 and 12 can be coupled to the
positive and negative (or ground, GND) DC Bus poles.
[0075] In the example of FIG. 3, the upper and lower converter arms
of the single AC phase have each four converter cells. Initially,
at start up of the multilevel converter 10, all the converter cells
are discharged and the load is disconnected, e.g. by switch 19. The
switches S.sub.C1 and S.sub.C2 of the resistor circuits 71, 72 are
opened, as illustrated in FIG. 3A. In the figure, S.sub.mn denotes
the switches of the converter cells, wherein "m" indicates the cell
number (m=1 to 8) and "n" denotes the switch in each cell (n=1 or
2, as illustrated in FIG. 2). In FIG. 3A, the topmost capacitor
C.sub.1 is charged. For this purpose, switch S.sub.11 of converter
cell 30 is closed and switch S.sub.12 is opened. In the remaining
converter cells, the switches S.sub.m2 (m=2 to 7) are closed. The
remaining cells m=2 to 8 are thus essentially bypassed. Accordingly
an RLC circuit is formed (including inductances L1 and L2), and the
capacitor C.sub.1 is charged to the DC Bus voltage V.sub.dc.
[0076] Generally, it is only required to charge the capacitor to a
voltage level of V.sub.dc/4. By means of e.g. a voltage sensor
provided in the converter cell which measures the charging state of
the capacitor, it can be determined when this voltage level is
reached, in response to which the switch S.sub.11 is opened and the
switch S.sub.12 is closed (as illustrated in FIG. 3B). Capacitor
C.sub.1 is thus charged to the desired level and taken out of the
circuit.
[0077] At the same time or with a short delay after taking out
C.sub.1, the capacitor C.sub.2 of converter cell 31 can be inserted
into the circuit by closing switch S.sub.21 and opening switch
S.sub.22 (see FIG. 3B). A RLC circuit similar to the one mentioned
above is thus formed, as the resistance and capacitance values in
the RLC circuit are essentially equal. The second capacitor C.sub.2
is thus also charged through the resistances R.sub.C1 and R.sub.C2
of the resistor circuits 71 and 72.
[0078] This charging process can be repeated until all the
capacitors in the upper and lower converter arms 61, 62 are charged
to V.sub.dc/4. The capacitors of the converter cells in further
converter legs can be charged with a corresponding process. For a
3-phase converter, converter legs 21, 22 and 23, as illustrated in
FIG. 1, may for example be energized, wherein the charging of the
capacitors of different converter legs can occur
simultaneously.
[0079] In the example of FIG. 3, the upper and lower converter arms
61 and 62 are essentially symmetric, with a resistor circuit 71 and
72 being provided in each converter arm. It should be clear that
other configurations are also conceivable. As an example, only one
resistor circuit may be provided, through which the capacitor of
each converter cell can be charged. The complexity may thus be
further reduced. On the other hand, providing two resistor circuits
71, 72 can be advantageous for discharging the capacitor of a
particular converter cell or for charging the capacitor of a
converter cell from the AC terminal 15. The charging in FIGS. 3A
and 3B occurs from the DC side, yet as will be illustrated further
below, the charging is also possible from the AC side (terminal
15). It should furthermore be clear that each converter arm 61, 62
may comprise more or fewer converter cells, and that the capacitors
of each converter cell may be charged to different voltage
levels.
[0080] FIG. 4 again illustrates the multilevel converter 10 of
FIGS. 3 and 3B. In FIG. 4, the charging of the capacitors occurs
with a method in accordance with another embodiment of the
invention. In FIG. 4, the capacitors of the converter cells can be
charged without the need for obtaining the charging level of the
capacitors by means of a voltage sensor. In the example of FIG. 4,
all capacitors of the upper converter arm 61 are charged
simultaneously. As illustrated, all switches S.sub.i1 (i=1 to 4)
are closed in the upper converter arm 61, and the switches S.sub.i2
(i=5 to 8) in the lower converter arm 62 are closed so that the
lower converter cells are bypassed. Thus, each of the capacitors in
the upper converter arm 61 is charged to a voltage of V.sub.dc/4,
yet the charging occurs with a different time constant compared to
a single capacitor.
[0081] The capacitors of the converter cells of the lower converter
arm 62 can be charged correspondingly by closing the switchers
S.sub.i2 of the upper converter arm and the switches S.sub.i1 of
the lower converter arm. The remaining switches of the converter
cells are opened. This method can be performed with any number of
converter cells in each converter arm, the capacitor of each
converter cell then being charged to a voltage of V.sub.dc/n
wherein n denotes the number of converter cells in the converter
arm.
[0082] In dependence on the values of the resistance R, the
inductance L and the capacitance C of the capacitor, the series RLC
circuit can have an under-damped response. This may cause currents
to flow into the DC Bus, which is generally undesirable.
Accordingly, the resistance R of the resistor circuit may be
adjusted so as to achieve an over-damped response of the RLC
circuit. The selection of the resistance for achieving the
over-damped response is outlined in detail further below. This can
ensure that energy is always drawn from the DC power source during
the charging process.
[0083] In the configuration of the multilevel converter 10 in
accordance with the above described embodiment, it is also possible
to selectively discharge the capacitors of the converter cells.
This is illustrated in FIG. 5. Two converter arms 21 and 22 of the
multilevel converter 10 for two different AC phases are illustrated
in FIG. 5. By closing a switch 80, the upper converter arms of both
converter legs 21 and 22 are shorted. The electrical links 41 and
42 (as illustrated in FIG. 1) are electrically connected to each
other. By opening the switches (or breakers) 19, 82 and 81, both
the AC or DC power supply and the DC or AC load, respectively, are
separated from the multilevel converter 10 (i.e. the DC and AC side
breakers are opened). As the operation of the multilevel converter
10 ceases, the capacitors may still be charged to a voltage of a
bout V.sub.dc/4. The capacitor to be discharged is now inserted
into the circuit by means of switches S.sub.1 and S.sub.2 of the
respective converter cell, while all the other converter cells are
bypassed. In the example of FIG. 5, the capacitor of converter cell
30 is inserted into the circuit and is discharged through the
inductances 18, and the resistor circuits 71 and 73. The AC circuit
formed this way may be over-damped or under-damped, the capacitor
being discharged in both cases.
[0084] Note that FIG. 5 only illustrates the upper converter arms
of multilevel converter 10, the lower converter arms not being
shown for the purpose of a comprehensive presentation.
[0085] As mentioned above, it is also conceivable to charge the
capacitors of the converter cells of the multilevel converter 10
from an AC power source connected thereto, as illustrated in FIG.
6. This may for example be performed for converters in a HVDC (high
voltage DC) application in which the converter generates a DC
voltage that is to be transmitted over a DC transmission line. The
converter thus acts as an AC-DC rectifier and the AC voltage source
is available for charging. Again, FIG. 6 only illustrates the two
upper arms of two converter legs 21, 22. Two phases of the AC power
source connected to the respective converter legs are shown. The
capacitor of the cell which is to be charged is now inserted into
the circuit. All other cells are bypassed. Depending on the instant
when the AC voltage is applied to the capacitor, the capacitor
voltage rises to the desired level. The capacitor may for example
be charged during a leading edge of the AC voltage waveform. Using
the above mentioned voltage sensor, the level to which the
capacitor is charged can be determined, and the capacitor can be
taken out of the circuit when reaching a voltage level of
V.sub.dc/n. This can be repeated for the remaining converter cells
so that the capacitor of each converter cell can be charged.
Attention has to be paid on the polarity of the capacitor when
charging from the AC power source.
[0086] As can be seen, in the examples outlined above, the
capacitors of the converter cells can be directly charged from the
DC or AC power source, without requiring an auxiliary voltage
source for charging. This is achieved by making use of the resistor
circuits using which the resistance can be inserted or taken out
(bypassed) of the converter leg. In the following, it will be
explained how an over-damped RLC circuit is obtained for avoiding
currents flowing into the power source.
[0087] In case of a series RLC circuit fed from a dc voltage
source, the governing differential equation is:
V dc = Ri ( t ) + L i ( t ) t + 1 C .intg. i ( t ) t ( 1 )
##EQU00001##
wherein R is the resistance, i(t) is the time (t) dependent current
through the circuit and d/dt is the time derivative. For obtaining
an over-damped response of this second order differential equation,
the damping factor .xi.>1 can be chosen, wherein the following
parameters are defined:
.xi.=.alpha./.omega..sub.0 (2)
.alpha.=R/2L (3)
.omega..sub.0=1/ {square root over (LC)} (4)
[0088] The solution to this equation for zero initial conditions is
given by
i ( t ) = V dc 2 L .omega. 0 .xi. 2 - 1 { - .omega. 0 ( .xi. - .xi.
2 - 1 ) t - - .omega. 0 ( .xi. + .xi. 2 - 1 ) t } ( 5 )
##EQU00002##
[0089] It is useful to define the following two terms:
.alpha..sub.1=.omega..sub.0(.xi.+ {square root over
(.xi..sup.2-1)}) (6)
.alpha..sub.2=.omega..sub.0(.xi.- {square root over
(.xi..sup.2-1)}) (7)
[0090] The peak value of the current (i.sub.peak) can be obtained
by differentiating (5) with respect to time. Thus,
i peak = V dc 2 L .omega. 0 .xi. 2 - 1 { - .alpha. 2 t peak - -
.alpha. 1 t peak } ( 8 ) ##EQU00003##
where, t.sub.peak is the time instant when this peak current
occurs.
t peak = ln ( .alpha. 1 / .alpha. 2 ) .alpha. 1 - .alpha. 2 ( 9 )
##EQU00004##
[0091] A plot of i.sub.peak with variation in .xi. following eqn.
(8) is shown in FIG. 7 where (V.sub.dc/2Lw.sub.0) is calculated
from the values shown in Table 1. It shows that for values of .xi.
close to unity, the peak current rises very sharply. However, for
.xi. equal to 20 or higher, the peak current does not change very
much. This parameter can be used to limit the charging current in
the converter and will be used for selecting the values of R as
described hereinafter.
TABLE-US-00001 TABLE I Parameters Value Parameters Value Rated
line-line 6.6 kV DC link voltage 10 kV voltage V.sub.dc Rated
current 583 A Maximum modulation 0.54 index Rated power 6 MW Number
n of cells 4 in each arm Rated power factor 0.9 Arm inductance 1 mH
Rated frequency 50 Hz Cell capacitance 8 mF
[0092] Table 1 shows some of the circuit parameters of a 6.6 kV, 6
MW MMC in accordance with an embodiment of the invention that is
adapted for a drives application. The values of capacitance of the
capacitor of the converter cells and arm inductance give good
performance at steady state condition (during normal operation).
The resistance of the resistor circuit of the converter arm can be
selected for this converter as follows. It is assumed that the
charging current will be limited to 500 A (this can certainly be
varied for different embodiments). From FIG. 7, a damping factor of
20 can be selected for achieving this maximum current. From eqns.
(2), (3) and (4), the value of the resistance in the converter arm
can be determined to 10 ohms. During discharge of the converter
cell as illustrated in FIG. 5, the peak discharge current is
limited to V.sub.dc/(8R) which is equal to 125 A and is an
acceptable value.
[0093] For the case shown in FIG. 4, where the capacitors of a
converter arm are charged simultaneously, the equivalent series
capacitance of the converter arm is 2 mF. A curve similar to FIG. 7
can be determined for this case (not shown), and the required
damping factor can be determined. The value of the resistance of
the resistor circuit in the converter arm can thus be determined to
10 ohms.
[0094] Two simulation results for charging the capacitors in a
converter arm individually or simultaneously are illustrated in
FIGS. 8 and 9. In FIGS. 8A and 8B, the charging method as outlined
with respect to FIG. 3 is followed. FIG. 8A shows the charging
current flowing in a converter arm. At t=0.1 s, the DC side breaker
(e.g. breaker 81) is closed and the top capacitor C1 is charged
towards V.sub.dc. Once the voltage on the top capacitor C.sub.1
reaches 2.5 kV, the capacitor is disconnected. The second and
subsequently all other capacitors in the arm are charged
sequentially (FIG. 8B). Once all the four capacitors are charged,
all S.sub.i1 (i=1 to 4) are closed and the complementary switches
are opened. This will immediately bring the current in the circuit
to zero. At this condition, the DC side breaker can be opened
easily. This completes the charging process for one converter arm.
The remaining converter arms may be charged correspondingly.
[0095] FIGS. 9A and 9B show the case in which all the capacitors of
a converter arm are charged simultaneously without making use of a
voltage sensor. This is similar to the previous case. The DC side
breaker 81 can be safely opened once the charging current dies down
to zero.
[0096] FIG. 10 shows a flow diagram of a method according to an
embodiment of the invention that may be performed by any of the
above mentioned multilevel converters 10. In step 101, the load and
the power source are disconnected from the multilevel converter. In
step 102, a resistance is connected in series with the converter
cells of a converter leg. For example the resistances R.sub.C1 and
R.sub.C2 may be inserted into the circuit of converter leg 21 by
opening the switches S.sub.C1 and S.sub.C2 of the respective
resistor circuits. Otherwise, the resistances are bypassed when
these switches are closed.
[0097] In step 103, the capacitor of a converter cell or the
capacitors of all converter cells of a converter arm are connected
in series with the resistance, as for example described with
respect to FIG. 3 or 4. This can be done by appropriately switching
the switches S.sub.1 and S.sub.2 of the converter cells.
[0098] In step 104, electric power is supplied to the multilevel
converter, e.g. by connecting an AC or DC power source thereto.
This can for example be achieved by closing the switch/breaker 19
or 81, respectively. The capacitor/all capacitors of the converter
arm that are connected in series with the resistor are charged from
the connected main power source through the resistance in step 105.
The charging is repeated for all the capacitors in the converter
leg (step 106), e.g. by charging each capacitor separately or by
charging the second converter arm at once.
[0099] Corresponding steps can certainly be repeated for further
converter legs, if such are provided in the multilevel converter.
The multilevel converter is now energized and can start normal
operation. For this purpose, the resistance in the converter leg is
bypassed (step 107), e.g. by closing the switch of the respective
resistor circuit. In step 108, the load is connected to the
multilevel converter (e.g. an AC drive of a DC transmission line),
whereafter the operation of the multilevel converter is started
(step 109). The multilevel converter can now operate in steady
state.
[0100] When stopping the operation of the multilevel converter, the
converter cells can be de-energized as outlined above by
discharging the capacitor of the converter cell via the converter
arms of two converter legs and through the resistance which is
again inserted into the respective converter leg for this
purpose.
[0101] Embodiments disclosed herein thus provide methods for
charging and discharging the capacitances of a multilevel
converter, in particular a modular multilevel converter. It should
be clear that the features of the embodiment described above may be
combined. The charging does not require any auxiliary voltage
source. In case of drives application, the DC bus can be used to
charge the capacitances of the converter cells. By inserting a
resistance into the converter leg and adjusting its value, the
capacitors of the converter cells can be charged to the desired
voltage level. The additional resistance can be bypassed under
normal operating conditions.
* * * * *